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FAQs 

Electrical

Strings
The cartoon above describes how panels are clipped together in 'series'..which simply means a positive to negative connection of the MC4 plugs on the back of every panel.

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As each panel is connected to the next, the total voltage increases but the current (AMPS) stays the same. We can keep adding panels until we reach the maximum input voltage for the inverter...normally between 400V and 600V for residential inverters...when we can't add any more in series without exceeding the inverter voltage max, we look at connecting strings in parallel.

The cartoon also explains that when two identical voltage strings are wired together in parallel, the voltage stops rising but the current (AMPS) doubles. This is important because some inverters can accept the current from two parallel strings  and others can't.

Three or more roof orientations
If you want solar panels on your North, West and East facing roof, conventional wisdom says that you will need micro inverters because regular "string inverters" can only handle panels on two roof orientations (2 x MPPT).

That's true to a point. However, if you have the same number of panels on two roof orientations (e.g. East and West, or North and West) you can connect the strings together in parallel and still have a totally different number of panels on the 3rd roof orientation.

However, most solar inverters can't handle the doubled current without some "clipping".

Clipping
If you look at the voltage and current produced by a string of panels controlled by an inverter MPPT, you'll see that the voltage produced by the panels gets up to their optimal level (VMP) very quickly and stays that way all day unless clouds pass by, but the current produced is a bell curve, gradually rising to a peak, typically at mid day on a North facing roof, and falling away as the power of the sun declines. If your installation has been designed so that the inverter maximum current limit has been exceeded (e.g. a parallel of two strings with 18 amps on a 16 amp limit), then at mid day, it's possible that the inverter will clip off the excess current, resulting in you losing some power production (Volts x Amps = Watts). The same applies if the designer has exceeded the maximum input voltage of the inverter. It won't harm the inverter, it just means that the excess is clipped off. For those who go down this path, the overall power production against losses from clipping is overwhelmingly in their favour.

How many panels can be connected to an inverter MPPT?

Exceeding the input voltage maximum of an inverter will result in clipping (see above) but not damage. However If you want to do it right, take the VOC (open circuit voltage) and add on 6% to account for a cold morning where the voltage will increase. Divide the max input voltage of the inverter MPPT, round down and that's the maximum number of panels you can connect. Doing it that way, you are hitting the inverter with the  maximum voltage it will ever see (not under load).

Start up voltage
An inverter MPPT needs more juice to start up, than it does to stay working. A typical scenario is a 150V start up and then 100V to maintain. To work out the minimum number of panels required for start up, divide the startup voltage of the MPPT by the VMP of the panel, round up, and add on extra panel.

e.g. Inverter requires 150 V start up and panel VMP = 31

150/31 = 4.83, rounded up = 5,  plus 1 = 6 panels

Is one solar inverter better than another?

In terms of efficiency (converting panel power to AC), no, they are all around the 98% mark these days. In terms of fun features like WIFI reporting, Goodwe, Fronius and SMA are very reliable and detailed reporting and a stable WIFI platform. The rest, less so.

In terms of reliability, if you buy "cheaper" Chinese then you typically have a 30% chance of failure within the normal 5 year warranty period, European made, 5%. The reason is simply because the Chinese made inverters use lower quality components... just simple things like relays that only cost a couple of bucks for a good one fail because of corners cut. Accountants in the manufacturing and design process ...Pfft ! Before we set you off now as a Chinese made hater, bear in mind that German SMA make all their single phase inverters in China now, and Goodwe, a 100% Chinese brand is a million miles away from the normal cheap rubbish...and pretty much every Tier 1 solar panel is 100% Chinese made. Best inverter, in this writer's opinion, that ticks every single box, is Austrian made Fronius.

Is one solar panel better than another?

After tens of thousands of solar readings sent in every few months by our solar customers since 2008, we can categorically say, in answer to this all important question that is debated left right and centre on the internet.... "not by much".

One of the most clever marketing tricks in solar has been the 'solar experts' on Whirlpool and other forums claim that cheap panels wlll fail 'after three to four years'. Genius if you think about it. Enough to put you off straight away, but enough time for the comment to be forgotten. Whilst, we agree that big brands are best, especially now as they cost about the same as the lesser known ones, the actual reality, is that the cheaper no name brand panels we installed in 2008, 2009, 2010, 2011 are all going just fine. We've replaced a total of 40 panels of ANY brand since 2008. How many panels do you think we've installed over that time? Yes. Panels are very reliable...and you are reading this from a solar retailer that pretty much uniquely has continously monitored each and every system we have installed.

Some people really don't get it.

Whilst solar batteries may ultimately make this paragraph redundant, the current reality is that, these days at least 50% of a typical 5-6kW solar production is surplus to the daytime consumption of the average house. It has been our experience that many people will spend a huge amount of time researching what the very best, most efficient panels are, and the maximum they can cram on their roof to maximise their solar production. We've had many people (mostly engineers of one flavour or another) go against our recommendations for a $3,500 system and insist on a $7,000 one (micros, etc).

The critical factor they all ignore completely is the importance of their own home's day time power consumption. ANY solar power production that exceeds the immediate needs of the house in daylight hours, gets exported back into the grid and results in a payment of 7 cents per kWh. These, now much maligned engineers, are hard at work all day, which means their house is not consuming much power during the day, apart from, maybe, weekends.

So, for example, obtaining an extra 1000kWh of annual power production by totally optimising the installation, at a cost of thousands of $$, results in 1000 x 7 cents = $70 a year. We try to explain that spending an extra $3500 to save $70 a year may not be exactly best financial practice, but heh, we'll take their money if they insist.

 

Real label on the back of a solar panel (Q.Cells)

30.77V x 8.29A = 255W maximum power

Positive and Negative leads coming from junction box (Q.Cells)

 

 




 

 

 

 

 

 


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